The MinElem unstructured surface mesher generate triangular meshes considering just the chordal error parameters defined in the Meshing branch of the preferences window. This implies the triangles may not have a good aspect ratio, but the mesh represents accurately the shape of the surfaces with as less elements as possible.
This kind of meshes are often used for visualization purposes, where the angular quality of the triangles is not so relevant.

First of all, reset the mesh properties already assigned in previous steps of the tutorial: select Mesh->Reset mesh data.
Select MinElem as the unstructured surface mesher in the Meshing->Structuration type branch of the Preferences window, and set the Default options in the Chordal error branch of it. Set to None the Automatic correct sizes parameter of Meshing->General branch in order to see the result of this mesher in a more exaggerated way. (Remember to click Apply in the preferences window in order to set the parameters.)
Then, click on Mesh->Generate to generate the mesh and set 1000 as general mesh size. The resulting mesh is depicted. It can be appreciated that the straight lines are meshed with only one element, and the planar surfaces present a mesh only connecting their contour elements. This is due to the fact that they have no curvature, so the chordal error of their elements is always zero, and there is no need to refine the mesh there in order to represent the shape of the geometry.

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